1. Field of the Invention
The present invention relates to a package for optical communications modules (hereinafter called a package) for use in optical communications, radio communications, etc.
2. Description of the Background Art
As shown in FIGS. 4(a) and 4(b), a conventional package 20 comprises a side plate 21, two alumina members 28 (the alumina member at the other side is not shown.), a bottom plate 22, and a lid 23, which are assembled by soldering to hermetically seal the device housed inside. The bottom plate 22 has an extra length on both sides to provide holes 27 for fixing the package 20 to a printed substrate or a heat sink (not shown) by screws.
Usually, the bottom plate 22 of the package 20 is made of a Cuxe2x80x94W alloy, having high thermal conductivity, in order to dissipate the heat generated inside the package effectively to the outside. The side plate 21 is usually made of an Fexe2x80x94Nixe2x80x94Co alloy (brand name Kovar, for instance), which has a coefficient of linear expansion close to that of the bottom plate 22.
Recently, however, the miniaturization and performance enhancement of electronic equipment have increased the consumption power of ICs used in electronic equipment and the output of light emitting diodes (LEDs) and laser diodes (LDs) used as the device for optical communications. This increase requires the package 20 that houses the device to increase its heat-dissipating power further. In order to meet this requirement, when the bottom plate 22 is made of a material that has higher thermal conductivity than the Cuxe2x80x94W alloy, such as SiC, the difference in the coefficient of linear expansion between the side plate 21 and the bottom plate 22 is increased. This is because whereas SiC has a smaller coefficient of linear expansion than the conventional material, the conventional material is still used as the side plate 21 because a suitable material that has a coefficient of linear expansion comparable to SiC is yet to be found.
If the package 20 has a considerable difference in the coefficient of linear expansion between the side plate 21 and the bottom plate 22, the following problems may arise:
(a) Warping of the bottom plate 22:
When the side plate 21 and the bottom plate 22 of the package 20 are soldered, the difference in the coefficient of linear expansion between the two plates produces a thermal distortion, producing a permanent warp of several to several tens of micrometers to the bottom plate 22. When the package 20 having this warp is fixed to a printed board or a heat sink through the fixing holes 27 by screws, the aforementioned warp is straightened by force. This breaks the optical coupling between the LD 24 and the optical fiber 26 via the lens 25, so that the light excited by the LD 24 cannot be transmitted satisfactorily to the outside through the optical fiber 26.
(b) Repeated thermal stresses applied to the bonding portion between the side plate 21 and the bottom plate 22:
When the device in the package 20 repeats heat generation by its operation and cooling by the discontinuation of its operation, the bonding portion between the side plate 21 and the bottom plate 22 is subjected to repeated thermal stresses. The repeated thermal stresses cause the bonding portion to produce minute cracks or other abnormalities, which in turn break the hermeticity of the package 20. As a result, the properties of the device will deteriorate and subsequently lose its reliability.
On the other hand, packages housing devices have been required to have further increased heat-dissipating quality. Consequently, this requires he bottom plate of a package to be made of a material that has higher thermal conductivity than the conventional one, increasing the difference in the coefficient of linear expansion between the side plate and the bottom plate.
An object of the present invention is to offer a package that will not break its hermetic seal or impair the optical coupling between the optical devices even when the package comprises a side plate and a bottom plate that are made of materials having a relatively great difference in the coefficient of linear expansion.
The package of the present invention is formed by combining a side plate and a bottom plate, each having a different coefficient of linear expansion, both provided with tenons at their bonding portion. The tenon portions are matched together, and the bonding portion is soldered. When materials having a different coefficient of linear expansion are bonded, the rise and reduction in temperature may cause the material to warp or the bonding portion to generate minute cracks resulting from the repeated thermal stresses. The foregoing tenon structure is to prevent these problems.
It is desirable that the tenons have a width not less than 1 mm and not more than 5 mm. The effect will increase when a large number of tenons having a small width are provided. However, it is desirable to provide tenons having a width not less than 1 mm considering the manufacturing difficulty and cost and not more than 5 mm considering the effect.
As described above, packages have been required to further increase their heat-dissipating quality. To meet this requirement, the bottom plate of a package must necessarily be made of a material having excellent thermal conductivity. As a result, materials having a considerable difference in the coefficient of linear expansion between the package""s side plate and bottom plate have been used. This may result in permanent warpage to the bottom plate after the soldering and generate cracks at the bonding portion resulting from repeated thermal stresses.
In order to solve these problems, the present invention offers a package that has a tenon structure at the bonding portion between the side plate and bottom plate. The package is assembled by soldering the combined tenon portions. The package suppresses the generation of the aforemeetioned warp, is free from cracks caused by repeated thermal stresses, has excellent performance, and therefore is highly reliable.